The present disclosure relates generally to composite structures and, more particularly, to systems and methods for designing and manufacturing composite structures.
Composite structures provide several advantages over metallic structures. For example, composite structures can be configured to provide high specific stiffness and high specific strength relative to metallic structures. Furthermore, composite structures can be tailored to provide a relatively high degree of strength and stiffness along a primary load path. The ability to tailor the strength and stiffness of composites may result in lightweight structures. In addition, composite materials may have improved fatigue resistance relative to metallic materials and are more resistant to corrosion.
Composite structures may be formed as a stack of relatively thin layers or plies that are laminated together. Each ply in the composite laminate may include fibers that serve as the primary load-carrying constituent. The composite material may be formed as unidirectional tape wherein the fibers in each ply are oriented parallel to one another and are held in position by a matrix constituent such as an epoxy resin. The matrix constituent may also redistribute loads between adjacent fibers.
The composite structure may be configured such that the fibers in one ply are oriented in the same direction of the composite structure or in a different direction than the fibers in adjacent plies. The relative orientations of the plies may be selected to provide the desired strength and stiffness characteristics of the composite structure. Each ply in the composite laminate may be formed of the same material system. However, composite structures may also be formed as hybrid structures containing plies formed of different materials to achieve a desired design objective.
Conventional methods of designing a composite structure include constructing a finite element model (FEM) of the structure and subjecting the FEM to virtual loads to determine the stresses and strains in the structure and to perform sizing of the structure to meet strength, stiffness and weight requirements. An FEM is typically comprised of a mesh of multiple finite elements. Each element may represent one or more components or sub-components of the composite structure.
The process of designing a composite structure may include optimizing several design variables. Such design variables may include the geometry of the components and subcomponents that make up the composite structure. The geometry may include the size (i. e., length, width, height) and the shape of the components and sub-components. Additional design variables that may be optimized in the design process include the ply arrangement for the composite laminates to meet strength, stiffness, weight and other requirements. In many applications, the performance of a composite airplane structure can be adequately described by its plate stiffnesses; the A, B and D matrices.
Conventional methods of designing composite laminates include a determination of the stacking sequence of the laminate, including a determination of the individual ply thickness, the fiber angle of each ply, and the relative location of the ply in the through-the-thickness direction. For certain structures, loading conditions may dictate a laminate thickness requiring a relatively large quantity of plies. For example, a wing panel of an aircraft may require up to one hundred or more plies of composite material, each of which requires the determination of the fiber angle and the ply thickness. As may be appreciated, a ply-by-ply determination of such a stacking sequence for relatively thick composite laminates adds many design variables to the design process, which significantly increases the complexity of the design process. A further set of design variables that may be included in designing a composite structure is the material system of the plies that make up the composite laminate. The design of composite structures requires the knowledge of the thickness, orientation and material system for each ply within the stacking sequence. In order to find the most efficient structure (often the lightest weight design), the best possible combination of these parameters needs to be obtained.
This can become computationally challenging especially when the number of plies becomes large. This is the case even when ply orientations are restricted to the four traditional ply orientations: 0°, ±45° and 90°. The challenge becomes greater when non-traditional laminates are allowed. A composite laminate having at least one ply with fiber angle which is not equal to any one of the traditional ply orientations is referred to herein as a “non-traditional” layup or laminate.
There exists a need in the art for systems and methods for optimizing a composite structure that can characterize stacking sequences having a large number of plies in a computationally efficient manner.